how to Remove Duplicate Charatcers From String (i.e. AAA BBB---AB)

import java.util.*;

//class RemoveDuplicateCharatcersFromString = Main9
class Main9
{
public static String removeDuplicates(String s) {
StringBuilder build = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
String st = s.substring(i, i + 1);
if (build.indexOf(st) == -1) {
build.append(st);
}
}
return build.toString();
}
public static void main(String[] args)
{
String str="AAA BBB ";
String newString=removeDuplicates(str);
System.out.println(newString);
}
}
out put:-
                 A B